The present invention relates to magnetic flux locking methods and apparatus. More particularly, the present invention relates to methods and apparatus for supplying a modulation signal to a modulation coil so as to compensate for variations in the magnetic flux which is guided to a superconducting loop of a superconducting quantum interference device (hereinafter referred to as SQUID) by an input coil when the SQUID is operated and housed in a casing which is cooled by a refrigerator to a temperature below the critical temperature for superconduction.
It is known that a SQUID is capable of detecting magnetic flux with extremely high sensitivity. With attention to this characteristic, a SQUID is applied to various apparatus which are used in various technical fields. A SQUID is classified as an rf-SQUID if it has only on Josephson junction (hereinafter referred to as JJ) and as a dc-SQUID if it has two JJs. The rf-SQUID was generally used in the past years, while the dc-SQUID is being widely used in recent years because two JJs having similar characteristics can be obtained due to improvements in thin film manufacturing engineering in recent years.
FIG. 7 is an electric diagram for explaining the principle of a dc-SQUID flux meter.
The dc-SQUID includes a superconducting loop 71 and two JJs 72 which are provided at predetermined positions on the superconducting loop 71. A bias current is supplied to the opposite positions on the superconducting loop 71 with respect to the JJs 72 by a constant current source 70. An input coil 73, which is interconnected with a pickup coil 74 for detecting the magnetic flux of an object under measurement, is provided at a closed position on the superconducting loop 71. A voltage is output from the opposite positions on the superconducting loop 71 with respect to the JJs 72, the output voltage is transformed by a voltage transformation transformer 75 and then is amplified by an amplifier 76. The amplified voltage is demodulated by a synchronous detector 78 based on the signal output from an oscillator 77, then the demodulated signal is integrated by an integrator 79 so as to output a voltage which is proportional to the exterior magnetic flux. Further, the output signal output from the integrator 79 and the signal output from the oscillator 77 are added by an adder 80. The added signal is transformed into a feedback current by a voltage-current transformer 81, and the feedback current is supplied to a modulation coil 82 so as to eliminate the exterior magnetic flux detected by the pickup coil 74.
When the dc-SQUID is employed, interlinkage magnetic flux cannot be measured because the transformation coefficient of magnetic flux to voltage cyclically alternates based on the size of the interlinkage magnetic flux (refer to FIG. 8). However, a magnetic flux locked loop (hereinafter referred to as a FLL) maintains the magnetic flux at a point having the highest transformation rate of magnetic flux to voltage. Consequently, the interlinkage magnetic flux of the superconducting loop 71 can be measured when the dc-SQUID is integrated into a FLL having the arrangement in FIG. 7. More particularly, the magnetic flux which has the same size, and polarity which is the reverse of the externally supplied magnetic flux to the superconducting loop 71 through the pickup coil 74 and input coil 73, is fed back by the modulation coil 82 so as to cancel the external magnetic flux. The external magnetic flux can be measured by monitoring the feedback current supplied to the modulation coil 82.
When SQUIDs and FLLs are provided for multichannel application, the number of electrical connection lines remarkably increases with the increase in the number of channels (when the number of channels is n, the number of lines is 6n). The increase in the number of lines causes thermal contact to be greatly defective. Thereby the connecting section becomes greatly enlarged. Disadvantages arise in that liquid helium loss becomes great when a refrigerating system using liquid helium is employed, and in that refrigerating capacity of the refrigerator should be increased when the refrigerating system using the refrigerator is employed. Disadvantages further arise in that the measurement accuracy for measuring the magnetic flux is lowered because the magnitude of crosstalk between channels increases to a considerable value due to the arrangement of plural magnetic flux measurement system in closed condition.